tests: Link mocks with libvirt.so

tests: Link mocks with libvirt.so

In a lot of our mocks (if not all of them) we use our internal
APIs (e.g. VIR_ALLOC). So far, we're relying on test binary that
links with the mock to drag in libvirt.so. Well, this works only
partially. Firstly, whatever binary we execute from tests will
fail (e.g. as Martin reported on the list ./qemucapsprobe fails
to execute qemu). Secondly, if there's a program that tries to
validate linking (like valgrind is doing) it fails because of
unresolved symbols.

Because of that we have to link our mocks with libvirt.so.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>